One of geostationary meteorological satellite systems has been described in "Progress in Astronautics and Aeronautics, vol. 97-Monitoring earth's ocean, land, and atmospheric from space/sensors, systems, and applications" published in 1985 by AIAA. In such a geostationary meteorological satellite system, there is practically adopted an apparatus of transmitting image data between a satellite and a ground center station, and from the satellite to ground data collecting stations including aircraft and ship. The satellite comprises a QPSK (Quadrature Phase Shift Keying) modulator of modulating digital image data signals obtained by observations of the satellite and random signals produced in a PN (Pseudo Noise) code generator, an up-converter of converting frequency of signals modulated in the modulator into a radio frequency, and an antenna by which the modulated signals of the radio frequency of signals modulated in the modulator into a radio frequency are radiated and by which image data signals processed in the ground center station are received. The ground center station comprises an antenna by which the modulated signals are received from the satellite and by which the processed image data signals are radiated, a down-converter of converting the radio frequency of the modulated signals into an original frequency thereof, a demodulator of demodulating the modulated signals to output analog image data signals, an image data processing means of processing the analog image data signals to produce the aforementioned processed image data signal after being digitized, a memory of temporalily storing the processed image data signals, a modulator of modulating the carrier wave with the processed image data signals read out of the memory, and an up-converter of converting frequency of the processed image data signals into a radio frequency. In addition, each of the ground data collecting stations comprises an antenna by which the processed image data signals are received, a down-converter of converting the radio frequency of the processed image data signals into the original frequency thereof, a demodulator of demodulating the processed digital image data signals to output processed analog image data signals, an image data processing means of further processing the processed analog image data signals, and a display means of diaplaying an image in accordance with the image data signals which are processed in the image data processing means.
In operation, observations are made in the satellite during a limited period, for instance, 30 ms in a predetermined period, for instance, 600 ms of rotating on its own axis to produce a series of digital image data signals, for instance, of data rate 14 Mbps. The digital image data signals are modulated in the modulator during the limited period of 30 ms, while the random signals produced in the PN code generator are modulated therein during the remaining period of 570 ms. The modulated signals of the image data signals and the random signals are converted in the up-converter to be of the radio frequency. Thereafter, the modulated signals are radiated by the antenna to the ground. In this occasion, the modulated signals are decreased in its power by a transmitting power decreasing means, if necessary.
The modulated signals are received in the ground center station by the antenna therein and converted in the down-converter to be of an original frequency. The modulated signals thus converted in regard to frequency are demodulated in the demodulator to be output as the analog image data signals which are to be processed in the image data processing means. The processed image data signals are stored temporalily into the memory after being digitized and read out thereof to be modulated in the modulator. The processed image data signals thus modulated are radiated from the ground center station to the satellite after being converted to be of a radio frequency, and are radiated again from the satellite to the ground. At the present operating stage of radiating the processed image data signals, image data signals to be processed are impossible to be transmitted from the satellite to the ground center station. On the other hand, the processed image data signals are not transmitted from the satellite to the ground data collecting stations when the image data signals to be processed are transmitted therefrom to the ground center station.
Thus, the processed image data signals are received in the ground data collecting stations wherein the processed image data signals are reconverted in the down-converter to be of the original frequency and demodulated in the demodulator. The processed image data signals thus demodulated are further processed in the image data processing means whereby an image is displayed on the display means in accordance with the further processed analog image data signals.
According to the apparatus of transmitting image data in a satellite system mentioned above, however, there can be found disadvantages as follows.
(1) The ground data collecting stations are impossible to collect the image data signals at the operating stage during which the image data signals to be processed are transmitted from the satellite to the ground center station. Therefore, the ground data collecting stations are placed under such an inconvenient situation for four hours a day in a case where the satellite station radiates image data signals to be processed to the ground center station eight times a day for the reason why the radiation takes approximately thirty minutes in each time. PA0 (2) If the ground data collecting stations want to receive image data signals to be processed directly from the satellite so as to be processed therein, an antenna must be much larger for the reason why the image data signals to be processed are of such a high speed data rate as, for instance, 14 Mbps thereby resulting in the increase of facility cost. PA0 (3) Transmitting power in the satellite is limited in its upper level to meet the requirement in regard to power flux density provided for in the Radio Regulation Act so that an antenna must be larger in the ground center station and data collecting stations, while a transmitting power decreasing means must be provided in the satellite. PA0 (4) Even in a ground data collecting station provided on a ship, an antenna must be larger. In addition, it is necessary that a tracking means which is precisely operated is provided therein for the reason why a ship is pitched or rolled on the sea. PA0 (1) Image data signals to be processed and processed image data signals can be transmitted simultaneously from a satellite to the ground, while the image data signals to be processed and processed image data signals thus transmitted can be separated in accordance with a predetermined bit error rate in each of ground data collecting stations even if the both image data signals are of the same carrier frequency for the reason why the image data signals to be processed are decreased in regard to data rate and spread in regard to spectrum after digitized image data signals are produced in accordance with observations in the satellite, while the processed image data signals are transmitted to the satellite after being encoded with error correction code in a ground center station. PA0 (2) In each of ground data collecting station, gain is increased in receiving the processed image data signals in accordance with an encoded gain thereof whereby an antenna becomes smaller for the reason why the processed image signals are encoded with error correction codes in the ground center station. PA0 (3) Even image data signals to be processed can be easily received by a smaller antenna so that image data can be easily utilized even in the ground data collecting stations for the reason why the image data signals to be processed are decreased in regard to data rate. PA0 (4) Processed image data signals are transmitted together with image data signals to be processed so that the time of distributing the processed image data signals is increased by approximately four hours a day as compared to the conventional method and apparatus as set forth in more detail later. PA0 (5) An antenna becomes smaller so that tracking means on a ship etc. can be simplified. PA0 (6) Image data signals are radiated from a satellite in the form of the spread of spectrum so that the image data signals are received on the ground to meet the requirement provided for in the Radio Regulation Act in regard to power flux density thereof. For the reason, a transmitting power decreasing means is unnecessary to be provided in a satellite so that the satellite can be light in its weight, small in its size, and increased in its reliability. PA0 (7) An antenna can be smaller in a ground center station by one third as compared to that of the conventional method and apparatus so that facilities can be decreased in its cost.